Wear-resistant silicon nitride member and method of manufacture thereof

ABSTRACT

The present invention provides a wear resistant member composed of silicon nitride sintered body containing 2–10 mass % of rare earth element in terms of oxide thereof as sintering agent, 2–7 mass % of MgAl 2 O 4  spinel, 1–10 mass % of silicon carbide, and 5 mass % or less of at least one element selected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of said silicon nitride sintered body is 1 vol. % or less, a three-point bending strength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m 1/2  or more. According to the above structure of the present invention, there can be provided a silicon nitride wear resistant member and a method of manufacturing the member having a high strength and a toughness property, and particularly excellent in rolling and sliding characteristics.

TECHNICAL FIELD

The present invention relates to a wear resistant member mainly composedof silicon nitride and a method of manufacturing the member, and moreparticularly to a silicon nitride wear resistant member and a method ofmanufacturing the member capable of exhibiting excellent wearresistance, particularly rolling life characteristics when the wearresistant member is used as rolling bearing member, and is suitable as amaterial for constituting a rolling bearing member requiring anexcellent durability, and the wear resistant member also having a highdensity equal to or higher than the conventional silicon nitridesintered body and a high mechanical strength which is inherent to thesilicon nitride sintered body, even if the sintered body is manufacturedthrough a sintering operation under a low temperature of 1600° C. orlower.

BACKGROUND ART

Various sintering compositions for the silicon nitride sintered bodiesare well known: such as silicon nitride/oxide of rare earthelement/aluminum oxide system; silicon nitride/yttrium oxide/aluminumoxide/aluminum nitride system; and silicon nitride/oxide of rare earthelement/aluminum oxide/titanium oxide system or the like. Sinteringassistant agents composed of the oxides of rare earth elements, such asyttrium oxide (Y₂O₃) in the sintering compositions listed above, have afunction of generating grain boundary phase (liquid phase) composed ofSi-rare earth element-Al—O—N or the like during the sintering operation.Therefore, the sintering assistant agents are added to a materialcomposition for enhancing the sintering characteristics of sinteringmaterials, and achieve high density and high strength of the sinteredbodies.

According to the conventional art, the silicon nitride sintered bodiesare generally mass-produced as follows. After a sintering assistantagent as mentioned above is added to the material powder of siliconnitride, the material mixture is molded to form a compact. Thus obtainedcompact is then sintered in a sintering furnace at a high temperature ofabout 1,700–1,900° C. for a predetermined period of time.

However, in the conventional manufacturing method described above, sincethe sintering temperature was greatly high to be about 1700 to 1900° C.,there had been raised the following problems. That is, it was requiredto upgrade a heat-resistant specification for the sintering furnace andancillary equipments thereof, so that an installation cost of themanufacturing facilities was greatly increased. Further, it wasdifficult to adopt a continuous manufacturing process, so that amanufacturing cost of the silicon nitride sintered body was remarkablyincreased and a mass-productivity of the sintered body wasdisadvantageously lowered.

In addition, although the silicon nitride sintered body produced by theconventional method achieves an improved bending strength, fracturetoughness and wear resistance, however, the improvement is insufficient.A durability as a rolling bearing member requiring a particularlyexcellent sliding property is insufficient, so that a furtherimprovement has been demanded.

In these days, a demand of ceramic material as precision device membershas increased. In these applications, advantages such as high hardnessand light weight together with high corrosion resistance and low thermalexpansion property of the ceramic are utilized. In particular, in viewof the high-hardness and high wear resistance, application as a wearresistant member for constituting a sliding portion of the bearing orthe like has been rapidly extended.

However, in a case where rolling balls of a bearing or the like wereconstituted by the wear resistant member composed of ceramic, when therolling balls were rolled while being repeatedly contacted withcounterpart at a high stress level, the rolling life of the wearresistant member was not sufficient yet. Therefore, a surface of thewear resistant member is peeled off and the member causes cracks, sothat the defective member was liable to causes vibration and damage to adevice equipped with the bearing. At any rate, there had been posed aproblem that the durability and reliability as a material forconstituting the parts of the device was low.

The present invention had been achieved for solving the aforementionedproblems. Accordingly, an object of the present invention is to providea wear resistant member and a method of manufacturing the memberexcellent in wear resistance, particularly excellent in rolling lifecharacteristics that are suitable as rolling bearing member, in additionto a high density equal to or higher than the conventional siliconnitride sintered body and a high mechanical strength which is inherentto the silicon nitride sintered body, even if the sintered body ismanufactured through a sintering operation under a low temperature of1600° C. or lower.

DISCLOSURE OF THE INVENTION

In order to attain the objects described above, the inventors of thepresent invention had studied the effects and influences of parameterssuch as the type of silicon nitride material powder, sintering assistantagent and additives, the amounts thereof, and the sintering conditionson the characteristics of the final products (i.e., the sintered bodies)by performing experiments.

As a result, the experiments provided the following findings. That is, asintering property was greatly improved, when certain amounts of a rareearth element, MgAl₂O₄ spinel or a mixture of magnesium oxide andaluminum oxide, silicon carbide, at least one element selected from thegroup consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr, were added to afine material powder of silicon nitride to prepare a material mixture.

Further, when the material mixture was molded and sintered at a lowtemperature of 1600° C. or lower; or when the sintered body aftercompletion of the sintering operation was further subjected to a hotisostatic pressing (HIP) treatment under predetermined conditions, awear resistant member composed of a silicon nitride sintered body havingan excellent wear resistance and a particularly excellent rolling lifeof sliding property could be obtained in addition to high density andhigh mechanical strength property that were equal to or higher thanthose of conventional silicon nitride sintered body.

The present invention had been achieved on the basis of theaforementioned findings.

That is, according to the present invention, there is provided a wearresistant member composed of silicon nitride sintered body containing2–10 mass % of rare earth element in terms of oxide thereof as sinteringagent, 2–7 mass % of MgAl₂O₄ spinel, 1–10 mass % of silicon carbide, and5 mass % or less of at least one element selected from the groupconsisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxidethereof, wherein a porosity of said silicon nitride sintered body is 1vol. % or less, a three-point bending strength is 900 MPa or more, and afracture toughness is 6.3 MPa·m^(1/2) or more.

Further, the same function and effect can be obtained even in a casewhere a mixture of magnesium oxide and aluminum oxide is added as anadditive component in place of MgAl₂O₄ spinel. Therefore, according tothe present invention, there is provided another wear resistant membercomposed of silicon nitride sintered body containing 2–10 mass % of rareearth element in terms of oxide thereof as sintering agent, 1–2 mass %of magnesium oxide, 2–5 mass % of aluminum oxide, 1–10 mass % of siliconcarbide, and 5 mass % or less of at least one element selected from thegroup consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxidethereof, wherein a porosity of said silicon nitride sintered body is 1vol. % or less, a three-point bending strength is 900 MPa or more, and afracture toughness is 6.3 MPa·m^(1/2) or more.

Further, in the above wear resistant member composed of silicon nitridesintered body, it is preferable that a maximum width of aggregatedsegregation existing in grain boundary phase of the silicon nitridesintered body is 5 μm or less.

Furthermore, it is preferable that an average width of the aggregatedsegregation existing in grain boundary phase of the silicon nitridesintered body is 2 μm or less.

When the silicon nitride material mixture is sintered, the sinteringassistant agent and compounds of the additive components are transformedinto liquid phase thereby to form grain boundary phase. When this liquidphase components in the grain boundary phase are aggregated andsegregated to become large, the mechanical strength of the sintered bodyis lowered. In particular, when the sintered body is used as the wearresistant member, the rolling property is disadvantageously lowered.Therefore, it is preferable that the sintered body has a fine structurein which the maximum width of the aggregated segregation existing ingrain boundary phase is 5 μm or less, and the average width of theaggregated segregation is 2 μm or less.

Further, the three point bending strength of the silicon nitridesintered body constituting above wear resistant member is 900 MPa ormore and a fracture toughness is 6.3 MPa·m^(1/2) or more. Therefore,there can be also formed a wear resistant member composed of the siliconnitride sintered body such that a rolling life defined as a rotationnumber of steel balls rolling along a circular track formed on the wearresistant member formed of the silicon nitride sintered body until asurface of the silicon nitride wear resistant member peels off is 1×10⁷or more, when the rolling life is measured in such a manner that acircular track having a diameter of 40 mm is set on the wear resistantmember, three rolling balls each having a diameter of 9.35 mm andcomposed of SUJ2 are provided on the circular track, and the rollingballs are rotated on the track at a rotation speed of 1200 rpm under acondition of being applied with a pressing load of 39.2 MPa.

Furthermore, in the above wear resistant member, it is preferable thatthe silicon nitride sintered body has a crash strength of 200 MPa ormore, and a rolling fatigue life defined as a time until a surface ofrolling balls composed of the silicon nitride wear resistant memberrolling along a circular track formed on a steel plate peels off is 400hours or more, when the rolling fatigue life is measured in such amanner that three rolling balls each having a diameter of 9.35 mm areformed from the silicon nitride wear resistant member, the three rollingballs are provided on the circular track having a diameter of 40 mm seton the steel plate formed of SUJ2, and the rolling balls are rotated ata rotation speed of 1200 rpm on the track under a condition of beingapplied with a pressing load so as to impart a maximum contact stress of5.9 GPa to the balls.

Further, in the wear resistant member according to the presentinvention, the silicon nitride sintered body contains at most 5 mass %of at least one element selected from the group consisting of Ti, Hf,Zr, W, Mo, Ta, Nb and Cr in terms of oxide thereof.

Further, when the wear resistant member composed of above the siliconnitride sintered body is a rolling bearing member, the wear resistantmember can exhibit particularly excellent sliding property anddurability.

Further, there is provided a method of manufacturing a wear resistantmember composed of silicon nitride sintered body comprising the stepsof: preparing a material mixture by adding 2–10 mass % of a rare earthelement in terms of the amount of an oxide thereof, 2–7 mass % ofMgAl₂O₄ spinel, 1–10 mass % of silicon carbide, and 5 mass % or less ofat least one element selected from the group consisting of Ti, Zr, Hf,W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a silicon nitridepowder containing 1.5 mass % or less of oxygen and 90 mass % or more ofα-phase type silicon nitride and having an average grain size of 1 μm orless; molding the material mixture to form a compact; and sintering thecompact in non-oxidizing atmosphere at a temperature of 1,600° C. orlower thereby to form a wear resistant member composed of siliconnitride sintered body.

In the above manufacturing method, the same function and effect can beobtained even in a case where a mixture of magnesium oxide and aluminumoxide is added as an additive component in place of MgAl₂O₄ spinel.Therefore, according to the present invention, there is provided anothermethod of manufacturing a wear resistant member composed of siliconnitride sintered body comprising the steps of: preparing a materialmixture by adding 2–10 mass % of a rare earth element in terms of theamount of an oxide thereof, 1–2 mass % of magnesium oxide, 2–5 mass % ofaluminum oxide, 1–10 mass % of silicon carbide, and 5 mass % or less ofat least one element selected from the group consisting of Ti, Zr, Hf,W, Mo, Ta, Nb and Cr in terms of oxide thereof, to a silicon nitridepowder containing 1.5 mass % or less of oxygen and 90 mass % or more ofα-phase type silicon nitride and having an average grain size of 1 μm orless; molding the material mixture to form a compact; and sintering thecompact in non-oxidizing atmosphere at a temperature of 1,600° C. orlower thereby to form a wear resistant member composed of siliconnitride sintered body.

In the above method of manufacturing the wear resistant member, it ispreferable that the method further comprises a step of: conducting a hotisostatic pressing (HIP) treatment to the silicon nitride sintered bodyin non-oxidizing atmosphere of 30 MPa or more at a temperature of 1,600°C. or lower after completion of the sintering step.

According to the above manufacturing method, the oxide of rare earthelement, MgAl₂O₄ spinel or the mixture of magnesium oxide and aluminumoxide, silicon carbide, and compound of Ti, Zr, Hf or the like are addedto the silicon nitride material powder when the silicon nitride sinteredbody constituting the wear resistant member is prepared. Therefore,MgAl₂O₄ spinel together with the oxide of rare earth element such asyttrium oxide or the like react with silicon nitride material powder togenerate the liquid phase having a low melting point and function assintering promoting agent, so that a densification of the molded bodycan be advanced at low temperature of 1600° C. or lower, and thesintering assistant agent exhibits a function of suppressing graingrowth in the crystal structure whereby the structure of the sinteredbody is made fine and the mechanical strength is improved.

Further, silicon carbide (SiC) is solely dispersed as particles in astructure of the sintered body, and has a function of significantlyimproving the rolling fatigue characteristics of the silicon nitridesintered body. On the other hand, the compound of Ti, Zr, Hf or the likepromotes the function as the sintering assistant agent of rare earthelement oxide or the like, and the compound also has a function ofdispersion-reinforcing the crystal structure as the same manner as SiC,thereby to improve the mechanical strength of the sintered body. As aresult, there can be obtained a wear resistant member composed ofsilicon nitride sintered body having a fine structure and excellence inmechanical properties such that a maximum width of aggregatedsegregation existing in grain boundary phase is 5 μm or less and anaverage width of aggregated segregation is 2 μm or less, the maximumsize of the pores is 0.4 μm or less, porosity is 1 vol. % or less, threepoint bending strength at room temperature is 900 MPa or more, fracturetoughness of 6.3 MPa·m^(1/2) or more and crush strength is 200 MPa ormore, and having excellent mechanical properties.

To achieve good sintering characteristic, high bending strength, highfracture toughness value and long rolling life of the product, thesilicon nitride fine powder which is used in the method of the inventionand contained as a main component in the sintered body constituting thewear resistant member of the invention preferably contains at most 1.7mass %, preferably, 0.7–1.5 mass % of oxygen and 90 mass % or more, morepreferably, 92–97 mass % of alpha-phase type silicon nitride, andfurther the powder has fine grains, that is, an average grain size of atmost 1 μm, more preferably about 0.4–0.8 μm.

By the way, as the silicon nitride material powder, two types of α-phasetype Si₃N₄ powder and β-phase type Si₃N₄ powder have been known.However, when a sintered body is formed from the α-phase type Si₃N₄powder, there is a tendency that a strength is liable to beinsufficient. In contrast, in case of the β-phase type Si₃N₄ powder,although a high temperature is required for the sintering operation,there can be obtained a sintered body having a high strength and astructure in which a number of silicon nitride fibers each having alarge aspect ratio are tangled in a complicate manner.

In this connection, in the present invention, since the sintered body isprepared by sintering α-phase type Si₃N₄ material powder at a lowtemperature of 1600° C. or lower, there can be obtained a sintered bodyin which α-phase type Si₃N₄ crystal grains and β-phase type Si₃N₄crystal grains are coexisting in the crystal structure. Accordingly,since a small amount of α-phase type Si₃N₄ crystal grains coexists amongthe β-phase type Si₃N₄ crystal grains, a substantial structure of acomposite material is formed, so that the strength and toughness valueof the sintered body can be improved.

In this invention, a reason why a blending amount of α-phase type Si₃N₄powder is limited to a range of 90 mass % (wt %) or more is as follows.That is, when the amount is set to a range of 90 mass % or more, abending strength, fracture toughness and rolling life of the Si₃N₄sintered body are greatly increased thereby to further improve theexcellent characteristics of the silicon nitride. On the other hand, theamount is limited to at most 97 mass % in view of the sinteringproperty. It is more preferable to set the range to 92–95 mass %.

As a result, in order to achieve a good sintering characteristic, highbending strength, high fracture toughness and long rolling life of theproduct, as a starting material powder of the silicon nitride, it ispreferable to use the silicon nitride fine powder containing at most 1.7mass %, preferably, 0.7–1.5 mass % of oxygen, and at least 90 mass % ofalpha-phase type silicon nitride, and further the powder has finegrains, that is, an average-grain size of at most 1 g m, more preferablyabout 0.4–0.8 μm.

In particular, the use of a fine powder of silicon nitride having anaverage grain size of 0.7 μm or less facilitates forming a densesintered body having a porosity of at most 1% by volume withoutrequiring a large amount of a sintering assistant agent. The porosity ofthe sintered body can be easily measured in accordance with aArchimedes' method.

A total oxygen content contained in the silicon nitride sintered bodyconstituting the wear resistant member of the present invention isspecified to 4.5 mass % or less. When the total oxygen content in thesintered body exceeds 4.5 mass %, a maximum size of the pore formed inthe grain boundary phase is disadvantageously increased, and the pore isliable to be a starting point of a fatigue failure, thereby to lower therolling (fatigue) life of the wear resistant member. A preferable rangeof the total oxygen content is 4 mass % or less.

By the way, the term “total oxygen content of the sintered body”specified in the present invention denotes a total amount in terms ofmass % of oxygen constituting the silicon nitride sintered body.Accordingly, when the oxygen exists in the silicon nitride sintered bodyas compounds such as metal oxide, oxidized nitride or the like, thetotal oxygen content is not an amount of the metal oxide (and oxidizednitride) but an amount of oxygen in the metal oxide (and the oxidizednitride).

The maximum pore size formed in the grain boundaries of the siliconnitride sintered body constituting the wear resistant member of thepresent invention is preferably specified to 0.4 μm or less. When themaximum pore size exceeds 0.4 μm, the pore is liable to particularly bea starting point of a fatigue failure, thereby to lower the rolling(fatigue) life of the wear resistant member. A preferable range of themaximum pore size (diameter) is 0.2 μm or less.

Examples of the rare earth element to be added as a sintering assistantagent to a silicon nitride powder are Y, Ho, Er, Yb, La, Sc, Pr, Ce, Nd,Dy, Sm and Gd. Such a rare earth element may be added to the siliconnitride powder in the form of an oxide thereof or a substance which ischanged into an oxide thereof during the sintering process. Two or morekinds of such oxide or substance may be added to the silicon nitridepowder in a combination manner, Such a sintering assistant agent reactswith the silicon nitride powder so as to form a liquid phase and therebyserves as a sintering promoter.

The amount of a sintering assistant agent to be added to the materialpowder is set to be within a range of from 2 to 10 mass % in terms ofthe amount of an oxide thereof. If the amount is less than 2 mass %, thesintered body fails to achieve a sufficiently high density and highstrength. In particular, when an element which has a large atomic weightlike lanthanoid is used as the rare earth element at above less amount,a sintered body having a relatively low strength and relatively lowthermal conductivity is formed.

On the other hand, if the amount is more than 10 mass %, an excessivelylarge portion of the grain boundary phase is formed, and the generationof pore is increased, thereby reducing the strength of the sinteredbody. For this reason, the amount of a sintering assistant agent iswithin the range described above. For the same reason described above,the more preferred range of the amount of a sintering assistant agent is3 to 8 mass %.

In the present invention, MgAl₂O₄ spinel together with rare earthelement oxide such as yttrium oxide or the like to be used as additioncomponents react with silicon nitride material powder to generate theliquid phase having a low melting point and function as sinteringpromoting agent, so that a densification of the sintered body at lowtemperature of 1600° C. or lower can be performed, and MgAl₂O₄ spinelexhibits a function of controlling and suppressing grain growth in thecrystal structure whereby the structure of Si₃N₄ sintered body is madefine and the mechanical strength is improved. Further, MgAl₂O₄ spinelfunctions to lower a transition temperature at which α-phase typesilicon nitride is transformed into β-phase type silicon nitride, sothat the densification is advanced at a low temperature. Therefore,α-phase type silicon nitride phase should be remained to some extent inthe crystal structure after the sintering operation whereby to increasestrength and fracture toughness value of the resultant sintered body.

The same function and effect can be obtained even in a case where amixture of magnesium oxide (MgO) and aluminum oxide (Al₂O₃) is added asadditive components in place of MgAl₂O₄ spinel. In this case, anaddition amount of MgO is specified to a range of 1–2 mass %. If theaddition amount of MgO is less than 1 mass %, the densification of thesintered body becomes insufficient. On the other hand, if the amount isexcessively large so as to exceed 2 mass %, the strength of the sinteredbody and the rolling fatigue characteristic as a wear resistant memberare disadvantageously lowered.

Further, an addition amount of Al₂O₃ is specified to a range of 2–5 mass%. If the addition amount of Al₂O₃ is less than 2 mass %, thedensification of the sintered body becomes insufficient. On the otherhand, if the amount is excessively large so as to exceed 5 mass %, thestrength of the sintered body and the rolling fatigue characteristic asa wear resistant member are disadvantageously lowered.

Furthermore, silicon carbide (SiC) to be used as another additioncomponent in the present invention is added within a range of 1–10 mass% for the purpose of being solely dispersed as particles in the crystalstructure, and for exhibiting a function of drastically improving therolling life characteristic of the silicon nitride sintered body. Inaddition, silicon carbide (SiC) is added for improving the mechanicalstrength such as bending strength and fracture toughness value or thelike of the Si₃N₄ sintered body.

When the addition amount of silicon carbide (SiC) is less than 1 mass %,the sintered body fails to achieve a sufficiently addition effect. Onthe other hand, when the addition amount is excessively large to exceed10 mass %, the densification of the sintered body becomes insufficientthereby to lower the bending strength of the sintered body. For thisreason, the addition amount of SiC is set to the range of 1–10 mass %,preferably to a range of 3–7 mass %. In particular, in order to securegood performances together with sintering property, strength and rollinglife, it is preferable to set the addition amount of SiC to a range of3.5–6 mass %.

In this regard, there exist two types of silicon carbides i.e., α-typeand β-type silicon carbides. However, both α-type and β-type siliconcarbides exhibit the same function and effect to each other.

Further, in the present invention, at least one element selected fromthe group consisting of Ti, Hf, Zr, W, Mo, Ta, Nb and Cr is also addedas another component at an amount of 5 mass % or less. These elements tobe used as another addition component are added to the silicon nitridematerial powder as oxides, carbides, nitrides, silicides and boridesthereof. These compounds promote the sintering assistant effect of therare earth element, and also function to further lower the transitiontemperature at which α-phase type silicon nitride is transformed intoβ-phase type silicon nitride, and also promote dispersion thereof in thecrystal structure so as to enhance the mechanical strength of thesilicon nitride (Si₃N₄) sintered body. Among them, compounds of Ti, Zrand Hf are particularly preferred.

If the addition amount of these compounds is less than 0.3 mass %, thesintered body fails to achieve a sufficiently addition effect. On theother hand, if the amount is excessively large so as to exceed 5 mass %,the mechanical strength and rolling life of the sintered body aredisadvantageously lowered. For this reason, the preferred range of theamount of these compounds contained is at most 5 mass %. In particular,the amount is more preferably set to a range of 0.5–3 mass %.

The above compounds such as Ti, Zr, Hf or the like also serve as lightblocking agents (light shielding agents). More specifically, they colorthe silicon nitride type ceramic sintered body black and thus providesit with an opacity.

Further, since the porosity of the sintered body has a great influenceon the rolling life and bending strength of the wear resistant member,so that the sintered body should be manufactured so as to provide theporosity of 1 vol. % or less. If the porosity exceeds 1% by volume, thepore to be a starting point of the fatigue failure is rapidly increased,thereby to lower the strength of the sintered body and shorten therolling life of the wear resistant member.

The silicon nitride sintered body constituting the wear resistant memberaccording to the present invention can be produced by, for example, thefollowing processes. A material mixture is prepared by addingpredetermined amount of a sintering assistant agent, MgAl₂O₄ spinel orthe mixture of magnesium oxide and aluminum oxide, silicon carbide, arequired additive such as an organic binder, and a compound of Ti or thelike, to a fine powder of silicon nitride which has a predetermined fineaverage grain size and contains very small amount of oxygen. Thematerial mixture is then molded into a compact having a predeterminedshape. As a method of molding the material mixture, conventional moldingmethods such as the die-pressing method or the doctor-blade method,rubber-pressing method, CIP (cold isostatic pressing) method or the likecan be applied.

In a case where the molded compact is prepared through the abovedie-press-molding method, in order to particularly form a grain boundaryhardly causing the pores or voids, it is preferable to set the moldingpressure for the material mixture to 120 MPa or more. When the moldingpressure is less than 120 MPa, there are easily formed portions(segregated portions) to which the compound of rare earth element ascomponent mainly constituting the grain boundary is agglomerated, andthe compact cannot be sufficiently densified, so that there is obtaineda sintered body with many crack-formations.

Further, the above agglomerated portion (segregated portion) in thegrain boundary is liable to become a starting point of fatigue failure,thus lowering the life and durability of the wear resistant member. Onthe other hand, when the molding pressure is set to an excessively largevalue so as to exceed 200 MPa, a durability of the molding die isdisadvantageously lowered, and it cannot be always said that theproductivity is good. Therefore, the above molding pressure ispreferably set to a range of 120–200 MPa.

Subsequent to the above molding process, the molded compact is heatedand maintained at 600–800° C. for 1–2 hours in a non-oxidizingatmosphere or at 400–500° C. for 1–2 hours in the air, therebydegreasing the compact, that is, thoroughly removing the organic bindercomponent added in the material mixture preparing process.

The degreased compact is then sintered by normal-pressure-sinteringmethod or pressured-sintering method at a temperature of 1,600° C. orlower for 0.5–10 hours in non-oxidizing atmosphere filled with inert gassuch as argon gas or nitrogen gas or hydrogen gas. As thepressured-sintering method, various press-sintering methods such as apressurized-atmosphere sintering method, hot-pressing method, HIP (hotisostatic pressing) method or the like can be utilized.

In addition, when the silicon nitride sintered body is further subjectedto a hot isostatic pressing (HIP) treatment under a temperaturecondition of 1,600° C. or lower in non-oxidizing atmosphere of 30 MPa orlower, an influence of the pore constituting a starting point of fatiguefailure of the sintered body can be further reduced, so that there canbe obtained a wear resistant member having a further improved slidingproperty and rolling life characteristics.

The silicon nitride wear resistant member produced by the above methodachieves a total oxygen content of 4.5 mass % or less, a porosity of 1%or less, a maximum pore size (diameter) of 0.4 μm or less and excellentmechanical characteristics, that is, a three-point bending strength (atroom temperature) of 900 MPa or greater.

Further, there can be also obtained a silicon nitride wear resistantmember having a crush strength of 200 MPa or more and a fracturetoughness of 6.3 MPa·m^(1/2) or more.

According to the silicon nitride wear resistant member and the method ofmanufacturing the member of the present invention, the material mixtureis prepared by adding the predetermined amounts of the rare earthelement, MgAl₂O₄ spinel or the mixture of magnesium oxide and aluminumoxide, silicon carbide, and compound of Ti, Zr, Hf or the like to thesilicon nitride material powder, so that the sintering property isgreatly improved. Therefore, even if the molded compact is sintered at alow temperature of 1600° C. or lower, there can be obtained a siliconnitride wear resistant member having an excellent wear resistance, ahigh density and a high mechanical strength that are equal to or higherthan those of conventional silicon nitride sintered body. In particular,the silicon nitride wear resistant member is suitable for a materialconstituting a rolling bearing member in view of its excellent rollinglife characteristics.

In other word, according to the wear resistant member of the presentinvention, the grain growth of the silicon nitride crystal grains can besuppressed by using the predetermined sintering assistant agent and bysetting the sintering temperature to 1600° C. or lower. Since the graingrowth can be effectively suppressed, a triple point formed among thesilicon nitride crystal grains can be minimized, so that it becomespossible to make a width of the grain boundary phase small.

Further, since the sintering temperature is set to a lower level of1600° C. or lower, the width of the grain boundary phase formed duringthe sintering process can be decreased. Simultaneously, an evaporationof the components of the grain boundary phase or impurities contained inthe grain boundary phase are prevented from being discharged to outside.Therefore, the generation of pores is suppressed and a maximum size(diameter) of pore can be minimized, so that there can be obtained awear resistant member excellent in rolling life characteristics anddurability. Accordingly, when a bearing device is prepared by using thiswear resistant member as rolling bearing member, good sliding/rollingcharacteristics can be maintained for a long time of period, and therecan be provided a rotation machine having excellent operationalreliability and durability. Further, as an example of anotherapplication, the wear resistant member of this invention can be appliedto various fields such as engine parts, various tool material, variousrails, various rollers or the like which require an excellent wearresistance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross sectional view showing a thrust-type rolling abrasion(wear) testing machine for measuring rolling life characteristics of awear resistant member according to one embodiment of the presentinvention.

EXPLANATION OF THE REFERENCE NUMERALS

1—machine body, 2—wear resistant member, 3—rolling steel ball, 4—guideplate, 5—driving rotation shaft, 6—retainer, 7—lubrication oil,8—rolling ball (made of silicon nitride), 9—bearing steel plate (SUJ2plate)

BEST MODE FOR EMBODYING THE INVENTION

Next, preferred embodiments of the silicon nitride wear resistant memberaccording to the present invention will be explained more concretely onthe basis of the following Examples and Comparative Examples.

EXAMPLES 1–3

A material powder mixture for Examples 1 was prepared by adding 5 mass %of Y₂O₃ (yttrium oxide) powder having an average grain size of 0.9 μm, 5mass % of MgAl₂O₄ spinel powder having an average grain size of 0.5 μm,5 mass % of β-phase type SiC (silicon carbide) powder having an averagegrain size of 0.8 μm, and 1 mass % of ZrO₂ (zirconium oxide) powderhaving an average grain size of 0.6 μm, as sintering assistant agents,to 86 mass % of Si₃N₄ (silicon nitride) material powder containing 1.3mass % of oxygen, and 97% of α-phase type silicon nitride, and having anaverage grain size of 0.55 μm, followed by wet-mixing the materials inethyl alcohol for 96 hours using pulverizing balls as pulverizationmedia formed of silicon nitride, and drying the mixture, thereby toprepare a material powder mixture.

After adding a predetermined amount of an organic binder and a solventto the material powder mixture, thereby to prepare a blended granulatedpowder. Then, the granulated powder was press-molded at a moldingpressure of 130 MPa, thereby to prepare a number of molded compacts eachhaving a dimension of 50 mm (length)×50 mm (width)×5 mm (thickness) assamples for measuring bending strength and a number of molded compactseach having a dimension of 80 mm (diameter)×6 mm (thickness) as samplesfor measuring rolling life.

Thereafter, thus prepared molded compacts were degreased in air-flowingatmosphere at temperature of 450° C. for 4 hours. Thereafter, thedegreased molded compacts were sintered and densified by holding thecompacts in a nitrogen gas (N₂) atmosphere under a pressure of 0.7 MPaat a temperature of 1550° C. for 6 hours thereby to prepare a number ofsilicon nitride wear resistant members of Example 1.

In addition, as Example 2, the manufacturing steps were repeated underthe same conditions as in Example 1 except that a hot isostatic pressing(HIP) treatment was performed to the sintered body obtained in Example 1in such a manner that the sintered body was heated and sintered innitrogen gas atmosphere of 100 MPa at a temperature of 1500° C. for 6hours, thereby to prepare a silicon nitride wear resistant member ofExample 2.

Further, as Example 3, the manufacturing steps were repeated under thesame conditions as in Example 2 except that 1.5 mass % of MgO (magnesiumoxide) powder having an average grain size of 0.5 μm and 3.5 mass % ofAl₂O₃ (aluminum oxide) having an average grain size of 0.8 μm were addedin place of MgAl₂O₄ spinel powder, thereby to prepare a silicon nitridewear resistant member of Example 3.

COMPARATIVE EXAMPLES 1–4

As Comparative Example 1, the manufacturing steps were repeated underthe same conditions as in Example 1 except that the SiC powder was notadded to the material mixture thereby to prepare silicon nitride wearresistant member of Comparative Example 1.

Further, as Comparative Example 2, a hot isostatic pressing (HIP)treatment was performed to the sintered body obtained in ComparativeExample 1 in such a manner that the sintered body was heated andsintered in nitrogen gas atmosphere of 100 MPa at a temperature of 1500°C. for 1 hour, thereby to prepare silicon nitride wear resistant memberof Comparative Example 2.

Furthermore, as Comparative Example-3, the manufacturing steps wererepeated under the same conditions as in Example 1 except that 5 mass %of Al₂O₃ (aluminum oxide) powder having an average grain size of 0.8 μmwas added in place of MgAl₂O₄ spinel powder, thereby to prepare siliconnitride wear resistant member of Comparative Example 3.

Still further, as Comparative Example 4, the manufacturing steps wererepeated under the same conditions as in Example 2 except that the Si₃N₄(silicon nitride) material powder containing 1.7 mass % of oxygen and 91mass % of α-phase type silicon nitride, and having an average grain sizeof 1.5 μm was used, thereby to prepare silicon nitride wear resistantmember of Comparative Example 4.

With respect to thus prepared silicon nitride wear resistant members ofExamples and Comparative Examples, porosity, maximum width and averagewidth of the agglomerated segregations in the grain boundary,three-point bending strength at room temperature, fracture toughness androlling life were measured. The fracture toughness was measured byNiihara system based on a micro-indentation method. The measured resultsare shown in table 1.

In addition, the porosity of the sintered body was measured byArchimedes' method, while the maximum width and average width of theaggregated segregations in the grain boundary phase was measured asfollows. Namely, three regions each having a unit area of 100μm-length×100 μm-width were arbitrarily set on a cross section of thesintered body constituting the wear resistant member, then an enlargedphotographic image (magnification of about 5000) was taken with respectto the regions by means of a scanning-type electron microscope (SEM).Among the aggregated segregations shown in the image, a segregationhaving the largest diameter was selected as a maximum size of theaggregated segregation. Concretely, the maximum width of the aggregatedsegregation was measured as a diameter of a minimum circlecircumscribing the triple-point region formed among the crystal grains.

Further, the average width of the aggregated segregations in the siliconnitride sintered body was calculated as an average value of thesegregation widths at 20 sites in the observation field.

In this regard, when the structure of the silicon nitride sintered bodyis observed through a magnified photograph taken by SEM or the like, theaggregated segregation is revealed and observed with a highlighted colorbrighter than that of ordinary grain boundary phase. For example, incase of a monochrome photograph, the silicon nitride crystal grains arerevealed with a blackish color, while the grain boundary phase isrevealed with a white color, and the aggregated segregation is revealedwith a highlighted white color. Therefore, the aggregated segregationcan be sharply and easily distinguished from the grain boundary phase.Further, if necessary, when an existence of rare earth element isconfirmed by EPMA, a concentration of the rare earth element is revealedwith a color darker than that of ordinary grain boundary phase, so thatthe respective constitutional elements can be distinguished from one toother by means of this analyzing method.

Furthermore, the three-point bending strength was measured as follows.That is, bending test pieces each having a dimension of 40 mm (length)×3mm (width)×4 mm (thickness) were prepared from the respective sinteredbodies. Then, the test piece was supported at a supporting span of 30mm, while a load-applying speed was set to 0.5 mm/min. Under theseconditions, the three-point bending strength was measured.

Further, the rolling characteristics of the respective wear resistantmembers were measured by using a thrust-type rolling abrasion testingmachine shown in FIG. 1. This testing machine is constituted bycomprising: a plate-shaped wear resistant member 2 disposed in a machinebody 1; a plurality of rolling steel balls 3 provided on an uppersurface of the wear resistant member 2; a guide plate 4 provided at anupper portion of these rolling steel balls 3; a driving rotation shaft 5connected to the guide plate 4; and a retainer 6 for regulating alocation interval of the rolling steel balls 3. A lubricating oil 7 forlubricating a rolling portion of the balls is poured into the machinebody 1. The above rolling steel balls 3 and the guide plate 4 are formedof high-carbon-chromium bearing steel (SUJ2) prescribed by JIS G 4805(Japanese Industrial Standard). As the above lubricating oil 7, paraffintype lubricating oil (viscosity at 40° C.: 67.2 mm²/S) or turbine oilcan be used.

The rolling life of the respective plate-shaped wear resistant membersof these embodiments were measured in such a manner that a circulartrack having a diameter of 40 mm was set on an upper surface of the wearresistant member 2, three rolling steel balls each having a diameter of9.35 mm and composed of SUJ2 were provided on the circular track, andthe rolling steel balls were rotated on the track at a rotation speed of1200 rpm under a condition of being applied with a pressing load of439.2 MPa and a condition of lubrication by an oil bath filled withturbine oil thereby to measure the rolling life defined as a rotationnumber of steel balls rolling along the circular track located on thewear resistant member formed of the silicon nitride sintered body untila surface of the silicon nitride wear resistant member 2 peeled off. Themeasuring results are shown in Table 1 hereunder.

TABLE 1 Width of Aggregated Segregation in Liquid Three-PointPhase(Grain Boundary Bending Fracture Porosity Phase) (μm) StrengthToughness Rolling Life Sample (%) Average Maximum (MPa) (MPa · m^(1/2))(rotations) Example 1 0.2 0.5 1 990 6.6   5 × 10⁷ Example 2 0.02 0.6 1.51100 6.9 >1 × 10⁸ Example 3 0.02 0.6 1.5 1080 6.9 >1 × 10⁸ Comparative0.2 3 6 900 6.1   2 × 10⁶ Example 1 Comparative 0.02 3.5 6.5 1020 6.2  6 × 10⁶ Example 2 Comparative 3.2 2.5 5.5 800 5.8   4 × 10⁵ Example 3Comparative 1.3 3 6 875 6.0   1 × 10⁶ Example 4

As is clear from the results shown in Table 1, in the respective siliconnitride wear resistant members of Examples, each of the sintered bodiesconstituting the wear resistant members was manufactured so as tocontain a predetermined amount of additive components, so that thegeneration of pores or voids was effectively suppressed and the maximumwidth of the aggregated segregation was decreased to be fine wherebythere could be obtained silicon nitride wear resistant members havinggood strength characteristics and excellent rolling life and durability.Further, although not shown in Table 1, the maximum size (diameter) ofpores formed in the grain boundary phases of the respective wearresistant members according to all Examples was 0.4 μm or less.

On the other hand, in Comparative Example 1 in which the SiC componentwas not contained, the amount of aggregated segregation of the liquidphase components was increased, thereby to disadvantageously lower thestrength characteristics and rolling life.

Further, in a case where the HIP treatment was performed and the SiCcomponent was not contained as in Comparative Example 2, although thethree-point bending strength was high, the effect of decreasing theaggregated segregation was insufficient thereby to shorten the rollinglife of the wear resistant member.

Furthermore, in case of Comparative Example 3 where only Al₂O₃ (aluminumoxide) powder was added in place of MgAl₂O₄ spinel powder, the porositywas disadvantageously large even if the sintering operation wassufficiently performed, and the width of the aggregated segregationbecame large, so that it was confirmed that both the strength androlling life were lowered.

Furthermore, in case of Comparative Example 4 where the oxygen contentin the silicon nitride material powder was excessively large, the amountof pores generated due to the high oxygen content was large and thewidth of the aggregated segregation was increased, so that it wasconfirmed that both the bending strength and rolling life were lowered.

Next, preferred embodiments of the silicon nitride wear resistant memberaccording to the present invention applied to rolling balls of a bearingmember will be explained more concretely on the basis of the followingExamples and Comparative Examples.

EXAMPLES 1B–3B and COMPARATIVE EXAMPLES 1B–4B

Each of the blended granulated powders as prepared in Examples 1–3 andComparative Examples 1–4 was packed in the molding die and pressedthereby to prepare spherical primary molded bodies. Then, each of theprimary molded bodies was subjected to a rubber pressing treatment at apressure of 100 MPa, thereby to respectively prepare spherically moldedbodies as samples each having a diameter of 11 mm for measuring crushstrength and rolling fatigue life.

Next, after the respective spherically molded bodies were subjected tothe degreasing treatment and sintering operation under the sameconditions as in the corresponding Examples and Comparative Examples,there by to prepare the densified sintered bodies of correspondingExamples and Comparative Examples. Further, thus obtained sinteredbodies were subjected to grinding work so as to provide a ball-shapehaving a diameter of 9.52 mm and a surface roughness of 0.01 μm-Rathereby to prepare the respective rolling balls for a bearing as wearresistant members of Examples 1B–3B and Comparative Examples 1B–4B.

In this connection, the above surface roughness was measured as anarithmetic average surface roughness (Ra) which can be obtained byscanning the surface on equator of the ball by means of aprofilometer-type surface roughness measuring device.

With respect to thus prepared rolling balls as the wear resistantmembers of Examples and Comparative Examples, porosity, maximum andaverage widths of aggregated segregations in the grain boundary phase,crush strength at room temperature (25° C.), fracture toughness valueand rolling fatigue life were measured.

In this connection, the rolling fatigue life of the respective wearresistant members were measured by using the thrust-type rollingabrasion testing machine shown in FIG. 1. By the way, in the previousExample 1 or the like, an item to be evaluated was a plate-shaped wearresistant member 2 while the balls rolling on the surface of the wearresistant member 2 were the rolling steel balls 3 composed of SUJ2.However, in order to evaluate the silicon nitride rolling balls 8 ofExamples 1B–3B and Comparative Examples 1B–4B, a bearing steel plate 9composed of SUJ2 was provided and assembled in place of the wearresistant member 2.

The rolling fatigue life of the respective rolling ball was measured insuch a manner that three rolling balls 8 each having a diameter of 9.52mm were formed from the silicon nitride wear resistant member, the threerolling balls 8 were provided on the circular track having a diameter of40 mm set on the upper surface of the steel plate 9 formed of SUJ2, andthe rolling balls 8 were rotated at a rotation speed of 1200 rpm on thetrack under a condition of being applied with a pressing load so as toimpart a maximum contact stress of 5.9 GPa to the balls 8 and alubricating condition using an oil bath filled with turbine oil, therebyto measure the rolling fatigue life defined as a time until a surface ofthe rolling balls 8 composed of the silicon nitride wear resistantmember peeled off. The measured results are shown in Table 2 hereunder.

TABLE 2 Width of Aggregated Segregation in Liquid Phase(Grain BoundaryCrush Fracture Rolling Phase) (μm) Strength Toughness Fatigue LifeSample Porosity (%) Average Maximum (MPa) (MPa · m^(1/2)) (hr) Example1B 0.2 0.5 1 220 6.6 >400 Example 2B 0.02 0.5 1.5 270 6.8 >400 Example3B 0.02 0.7 1.5 260 6.9 >400 Comparative 0.2 3 6 200 6.1 250 Example 1BComparative 0.02 3.5 6.5 240 6.2 300 Example 2B Comparative 3.4 2.5 5.5185 5.8 100 Example 3B Comparative 1.3 3 6 190 6.0 200 Example 4B

As is clear from the results shown in Table 2, in the respective siliconnitride rolling balls of Examples, each of the sintered bodiesconstituting the rolling balls was manufactured so as to containpredetermined additive components, so that the generation of the poreswas effectively suppressed thereby to decrease the width of aggregatedsegregations in the grain boundary to be small. Therefore, there couldbe obtained the silicon nitride rolling balls each having a high crushstrength and an excellent durability such that the rolling fatigue lifeexceeds 400 hours.

On the other hand, in Comparative Example 1B in which SiC was notcontained in the balls, a large amount of pores remained in the sinteredbody thereby to disadvantageously lower the crush strength and therolling fatigue life. 115 Further, in a case where the HIP treatment wasperformed and the SiC component was not contained as in ComparativeExample 2B, the effect of minimizing the pore size was observed.However, the rolling fatigue life of the wear resistant member wasshortened.

Furthermore, in case of Comparative Example 3B where only Al₂O₃ wascontained in place of MgAl₂O₄ spinel, the porosity was disadvantageouslyincreased even if the sintering operation was sufficiently performed, sothat it was confirmed that both the crush strength and rolling fatiguelife of the wear resistant member were lowered.

Furthermore, in case of Comparative Example 4B where the silicon nitridematerial powder having an excessively large content of oxygen was used,the amounts of liquid phase component and pores generated due to thehigh oxygen content were large, so that it was confirmed that any of theporosity, crush strength, fracture toughness value, and rolling fatiguelife was insufficient.

In this connection, when the rolling fatigue life of the silicon nitriderolling balls were measured, three rolling balls each having a diameterof 9.52 mm were used. However, even if other balls having differentdiameters were selected or the number of balls to be provided waschanged, it was also confirmed that rolling properties in accordancewith the load conditions or the rolling conditions could be obtained.

Next, with respect to a plate-shaped wear resistant member preparedthrough other compositions or treating conditions than those of theprevious Examples will be explained more concretely with reference tothe following Examples and Comparative Examples.

EXAMPLES 4–35

Material mixtures for Examples 4–35 were prepared so as to providecomposition ratios shown in Tables 34 by blending Y₂O₃ powder, MgAl₂O₄spinel powder, SiC powder used in Example 1, oxide powders of variousrare earth elements having average grain sizes of 0.9–1 μm as shown inTables 3–4, MgO powder having an average grain size of 0.5 μm. Al₂O₃powder AlN powder having an average grain size of 1 μm, and powders ofvarious compounds having average grain sizes of 0.4–0.5 μm with Si₃N₄(silicon nitride) material powder used in Example 1.

After thus obtained respective material mixtures were subjected to themolding/degreasing operations under the same conditions as in Example 1,the compacts were subjected to the sintering operation and HIP treatmentunder the conditions shown in Tables 3–4, thereby to prepare a number ofsilicon nitride wear resistant members of Examples 4–35.

COMPARATIVE EXAMPLES 5–14

Material mixtures of Comparative Examples 5–14 were respectivelyprepared as indicated in Tables 34. More specifically, an excessivelysmall amount or an excessive amount of various additives such as oxideof rare earth element such as Y₂O₃ or the like, MgAl₂O₄ spinel, SiC orthe like were added thereby to prepare the material mixtures for therespective Comparative Examples.

After thus obtained respective material mixtures were subjected to themolding/degreasing operations under the same conditions as in Example 4,the compacts were subjected to the sintering operation and HIP treatmentunder the conditions shown in Tables 34, thereby to manufacture a numberof silicon nitride wear resistant members of Comparative Examples 5–14.

With respect to thus prepared plate-shaped silicon nitride wearresistant members of Examples and Comparative Examples, porosity,maximum width and average width of the aggregated segregations formed inthe grain boundary phase (liquid phase), three-point bending strength atroom temperature, fracture toughness and rolling life of the circularplates were measured under the same conditions as in Example 1. Themeasured results are shown in Tables 3–4 hereunder.

TABLE 3 Material Composition (mass %) Sintering Condition HIP ConditionRare Earth MgAl₂O₄ Temp. × Time × Press. Temp. × Time × Press. SampleSi₃N₄ Oxide Spinel SiC Other (° C.) × (hr) × (MPa) (° C.) × (hr) × (MPa)Example  4 89 Y₂O₃ 5 5 1 1550 × 6 × 0.01 1500 × 1 × 100  5 85 Y₂O₃ 5 5 51550 × 6 × 0.01 1500 × 1 × 100  6 80 Y₂O₃ 5 5 10 1550 × 6 × 0.01 1500 ×1 × 100  7 90 Y₂O₃ 2 5 3 1600 × 6 × 0.01 1500 × 1 × 100  8 82 Y₂O₃ 10 53 1600 × 6 × 0.01 1500 × 1 × 100  9 90 Y₂O₃ 5 2 3 1600 × 6 × 0.01 1500 ×1 × 100 10 83 Y₂O₃ 5 7 5 1550 × 6 × 0.01 1500 × 1 × 100 11 84 Y₂O₃ 5 5 5TiO₂ 1 1550 × 6 × 0.01 1500 × 1 × 100 12 83 Y₂O₃ 5 5 5 TiO₂ 2 1550 × 6 ×0.01 1500 × 1 × 100 13 80 Y₂O₃ 5 5 5 TiO₂ 5 1550 × 6 × 0.01 1500 × 1 ×100 14 83 Y₂O₃ 5 5 5 ZrO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 15 83 Y₂O₃ 55 5 HfO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 16 83 Y₂O₃ 5 5 5 WC 2 1550 × 6× 0.01 1500 × 1 × 100 17 83 Y₂O₃ 5 5 5 MO₂C 2 1550 × 6 × 0.01 1500 × 1 ×100 18 83 Y₂O₃ 5 5 5 Ta₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 19 83 Y₂O₃ 55 5 Nb₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 20 83 Y₂O₃ 5 5 5 Cr₂O3 2 1550× 6 × 0.01 1500 × 1 × 100 21 83 Y₂O₃ 5 5 5 TiO₂ 1 1550 × 6 × 0.01 1500 ×1 × 100 ZrO₂ 1 22 83 Y₂O₃ 5 5 5 TiO₂ 1 1600 × 6 × 0.01 None ZrO₂ 1 23 85CeO₂ 5 5 5 1550 × 6 × 0.01 1500 × 1 × 100 24 83 Er₂O₃ 7 5 5 1550 × 6 ×0.01 1500 × 1 × 100 25 83 Nd₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 2683 Sm₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 27 83 Ho₂O₃ 7 5 5 1550 × 6× 0.01 1500 × 1 × 100 28 83 Yb₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 10029 83 Dy₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 Comparative Example  589 Y₂O₃ 1 5 5 1550 × 6 × 0.01 1500 × 1 × 100  6 75 Y₂O₃ 15 5 5 1550 × 6× 0.01 1500 × 1 × 100  7 89 Y₂O₃ 5 1 5 1550 × 6 × 0.01 1500 × 1 × 100  881 Y₂O₃ 5 9 5 1550 × 6 × 0.01 1500 × 1 × 100  9 75 Y₂O₃ 5 5 15 1550 × 6× 0.01 1500 × 1 × 100 10 78 Y₂O₃ 5 5 5 TiO₂ 7 1550 × 6 × 0.01 1500 × 1 ×100 Width of Aggregated Rolling Segregation in Liquid Three-Point LifePhase(Grain Boundary Bending Fracture of Circular Porosity Phase) (μm)Strength Toughness Plate Sample (%) Average Maximum (MPa) (MPa ·m^(1/2)) (rotations) Example  4 0.01 1 2 1050 6.8   6 × 10⁷  5 0.02 0.61.5 1100 6.9 >1 × 10⁸  6 0.02 0.4 1 1035 6.5 >1 × 10⁸  7 0.08 1.5 2.51000 6.5 >1 × 10⁸  8 0.02 1.5 2.5 980 6.6 >1 × 10⁸  9 0.08 0.5 1 10006.8 >1 × 10⁸ 10 0.02 1 2 1050 6.6 >1 × 10⁸ 11 0.01 0.5 1 1125 6.8 >1 ×10⁸ 12 0.01 0.4 1 1110 6.8 >1 × 10⁸ 13 0.01 0.4 1 1000 6.5 >1 × 10⁸ 140.01 0.6 1.5 1100 6.8 >1 × 10⁸ 15 0.01 0.5 1 1090 6.9 >1 × 10⁸ 16 0.010.5 1 1040 6.7 >1 × 10⁸ 17 0.01 0.5 1 1085 6.8 >1 × 10⁸ 18 0.01 0.5 11040 6.8 >1 × 10⁸ 19 0.01 0.5 1 1040 6.8 >1 × 10⁸ 20 0.01 0.5 1 10006.7 >1 × 10⁸ 21 0.01 0.5 1 1130 7.0 >1 × 10⁸ 22 0.2 0.3 1 1050 6.9   5 ×10⁷ 23 0.01 0.6 1.5 1020 6.8 >1 × 10⁸ 24 0.01 0.5 1.5 1100 7.0 >1 × 10⁸25 0.01 0.5 1 1070 6.5 >1 × 10⁸ 26 0.01 0.5 1.5 1050 6.5 >1 × 10⁸ 270.01 0.5 1 1080 6.7 >1 × 10⁸ 28 0.01 0.5 1 1010 6.7 >1 × 10⁸ 29 0.01 0.51.5 1050 6.8 >1 × 10⁸ Comparative Example  5 0.2 0.7 1.5 890 5.9   6 ×10⁶  6 0.05 2 4 880 5.8   1 × 10⁶  7 1.5 0.5 1 890 6.4   2 × 10⁶  8 0.011 2 900 6.3   8 × 10⁶  9 1.5 0.3 1 850 5.8   2 × 10⁶ 10 0.02 0.7 1.5 8906.3   5 × 10⁶

TABLE 4 Plate-Shaped Wear Resistant Member Material Composition (mass %)Sintering Condition HIP Condition Rare Earth Temp. × Time × Press. Temp.× Time × Press. Sample Si₃N₄ Oxide Al₂O₃ MgO SiC Other (° C.) × (hr) ×(MPa) (° C.) × (hr) × (MPa) Example 30 86 Y₂O₃ 5 2 2 5 1550 × 6 × 0.011500 × 1 × 100 31 84 Y₂O₃ 5 5 1 5 1550 × 6 × 0.01 1500 × 1 × 100 32 84Y₂O₃ 5 4 2 5 1550 × 6 × 0.01 1500 × 1 × 100 33 86 Y₂O₃ 5 4 2 3 1550 × 6× 0.01 1500 × 1 × 100 34 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 1 1550 × 6 × 0.01 1500× 1 × 100 ZrO₂ 1 35 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 1 1550 × 6 × 0.01 None ZrO₂1 Comparative Example 11 88 Y₂O₃ 5 1 1 5 1550 × 6 × 0.01 1500 × 1 × 10012 81 Y₂O₃ 5 7 2 5 1550 × 6 × 0.01 1500 × 1 × 100 13 86.5 Y₂O₃ 5 3 0.5 51550 × 6 × 0.01 1500 × 1 × 100 14 81 Y₂O₃ 5 5 4 5 1550 × 6 × 0.01 1500 ×1 × 100 Width of Aggregated Rolling Segregation in Liquid Three-PointLife Phase(Grain Boundary Bending Fracture of Circular Porosity Phase)(μm) Strength Toughness Plate Sample (%) Average Maximum (MPa) (MPa ·m^(1/2)) (rotations) Example 30 0.01 0.3 1 1100 6.9 >1 × 10⁸ 31 0.01 0.51.5 1070 6.6 >1 × 10⁸ 32 0.01 0.6 1.5 1080 6.7 >1 × 10⁸ 33 0.01 0.8 1.51120 7.0 >1 × 10⁸ 34 0.01 0.4 1 1110 6.9 >1 × 10⁸ 35 0.3 0.3 1 1000 6.8  4 × 10⁷ Comparative Example 11 1.5 0.5 1 880 6.2   2 × 10⁶ 12 0.01 0.82 900 6.2   7 × 10⁶ 13 1.2 0.5 1 850 6.5   4 × 10⁶ 14 0.01 1.8 3.5 8906.4   5 × 10⁶

As is clear from the results shown in Tables 3–4, in the respectivesilicon nitride wear resistant members of Examples each of which wasmanufactured in such a manner that the material powder mixturecontaining specified additives was molded and sintered, followed bybeing subjected to HIP treatment as occasion demanded after completionof the sintering process, the generation of the pore was effectivelysuppressed thereby to enable the width of the aggregated segregationsformed in the grain boundary phase to be extremely small. Therefore,there could be obtained the silicon nitride wear resistant membershaving good strength characteristics and an excellent durability suchthat the rolling life exceeded 10⁸ for most cases of Examples.

On the other hand, in the silicon nitride sintered bodies as shown inComparative Examples 5–14 in which the amount of additives such as rareearth components were set to outside the range specified in the presentinvention, even if the sintering operation or the HIP treatment afterthe sintering operation were sufficiently performed, the rolling life ofthe wear resistant members were lowered. Further, it was confirmed thatthe sintered bodies of Comparative Examples could not satisfy at leastone of the required characteristics such as porosity, width of theaggregated segregation, three-point bending strength, fracture toughnessvalue or the like that were specified in the present invention.

Next, preferred embodiments of the wear resistant members of the aboveExamples 4–35 and Comparative Examples 5–14 applied to rolling balls ofa bearing member will be explained more concretely on the basis of thefollowing Examples and Comparative Examples.

EXAMPLES 4B–35B and COMPARATIVE EXAMPLES 5B–14B

Each of the blended granulated powders as prepared in Examples 4–35 andComparative Examples 5–14 was packed in the molding die and pressedthereby to prepare spherical primary molded bodies. Then, each of theprimary molded bodies was subjected to a rubber pressing treatment at apressure of 100 MPa, thereby to respectively prepare spherically moldedbodies as samples each having a diameter of 11 mm for measuring crushstrength and rolling fatigue life.

Next, after the respective spherically molded bodies were subjected tothe degreasing treatment under the same conditions as in Example 1, thedegreased bodies were treated under the conditions of the sinteringoperations and HIP treatments as shown in tables 5–6. Further, thusobtained sintered bodies were subjected to grinding work so as toprovide a ball-shape having a diameter of 9.52 mm and a surfaceroughness of 0.01 μm-Ra thereby to prepare the respective rolling ballsfor a bearing as wear resistant members of Examples 4B–35B andComparative Examples 5B–14B.

In this connection, the above surface roughness was measured as anarithmetic average surface roughness (Ra) obtained by scanning thesurface on equator of the ball by means of a profilometer-type surfaceroughness measuring device.

With respect to thus prepared rolling balls as the wear resistantmembers of the respective Examples and Comparative Examples, porosity,width of the aggregated segregation, crush strength, fracture toughnessvalue and rolling fatigue life were measured as the same manner as inExample 1B, The measured results are shown in Tables 5–6 hereunder.

TABLE 5 Rolling Ball-Shaped Wear Resistant Member Material Composition(mass %) Sintering Condition HIP Condition Rare Earth MgAl₂O₄ Temp. ×Time × Press. Temp. × Time × Press. Sample Si₃N₄ Oxide Spinel SiC Other(° C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example  4B 89 Y₂O₃ 5 5 11550 × 6 × 0.01 1500 × 1 × 100  5B 85 Y₂O₃ 5 5 5 1550 × 6 × 0.01 1500 ×1 × 100  6B 80 Y₂O₃ 5 5 10 1550 × 6 × 0.01 1500 × 1 × 100  7B 90 Y₂O₃ 25 3 1600 × 6 × 0.01 1500 × 1 × 100  8B 82 Y₂O₃ 10 5 3 1600 × 6 × 0.011500 × 1 × 100  9B 90 Y₂O₃ 5 2 3 1600 × 6 × 0.01 1500 × 1 × 100 10B 83Y₂O₃ 5 7 5 1550 × 6 × 0.01 1500 × 1 × 100 11B 84 Y₂O₃ 5 5 5 TiO₂ 1 1550× 6 × 0.01 1500 × 1 × 100 12B 83 Y₂O₃ 5 5 5 TiO₂ 2 1550 × 6 × 0.01 1500× 1 × 100 13B 80 Y₂O₃ 5 5 5 TiO₂ 5 1550 × 6 × 0.01 1500 × 1 × 100 14B 83Y₂O₃ 5 5 5 ZrO₂ 2 1550 × 6 × 0.01 1500 × 1 × 100 15B 83 Y₂O₃ 5 5 5 HfO₂2 1550 × 6 × 0.01 1500 × 1 × 100 16B 83 Y₂O₃ 5 5 5 WC 2 1550 × 6 × 0.011500 × 1 × 100 17B 83 Y₂O₃ 5 5 5 MO₂C 2 1550 × 6 × 0.01 1500 × 1 × 10018B 83 Y₂O₃ 5 5 5 Ta₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 19B 83 Y₂O₃ 5 55 Nb₂O5 2 1550 × 6 × 0.01 1500 × 1 × 100 20B 83 Y₂O₃ 5 5 5 Cr₂O3 2 1550× 6 × 0.01 1500 × 1 × 100 21B 83 Y₂O₃ 5 5 5 TiO₂ 1 1550 × 6 × 0.01 1500× 1 × 100 ZrO₂ 1 22B 83 Y₂O₃ 5 5 5 TiO₂ 1 1600 × 6 × 0.01 None ZrO₂ 123B 85 CeO₂ 5 5 5 1550 × 6 × 0.01 1500 × 1 × 100 24B 83 Er₂O₃ 7 5 5 1550× 6 × 0.01 1500 × 1 × 100 25B 83 Nd₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 ×100 26B 83 Sm₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 27B 83 Ho₂O₃ 7 5 51550 × 6 × 0.01 1500 × 1 × 100 28B 83 Yb₂O₃ 7 5 5 1550 × 6 × 0.01 1500 ×1 × 100 29B 83 Dy₂O₃ 7 5 5 1550 × 6 × 0.01 1500 × 1 × 100 ComparativeExample  5B 89 Y₂O₃ 1 5 5 1550 × 6 × 0.01 1500 × 1 × 100  6B 75 Y₂O₃ 155 5 1550 × 6 × 0.01 1500 × 1 × 100  7B 89 Y₂O₃ 5 1 5 1550 × 6 × 0.011500 × 1 × 100  8B 81 Y₂O₃ 5 9 5 1550 × 6 × 0.01 1500 × 1 × 100  9B 75Y₂O₃ 5 5 15 1550 × 6 × 0.01 1500 × 1 × 100 10B 78 Y₂O₃ 5 5 5 TiO₂ 7 1550× 6 × 0.01 1500 × 1 × 100 Width of Aggregated Segregation in LiquidRolling Phase(Grain Boundary Crush Fracture Fatigue Porosity Phase) (μm)Strength Toughness Life of Sample (%) Average Maximum (MPa) (MPa ·m^(1/2)) Ball (hr) Example  4B 0.01 1 2 235 6.8 >400  5B 0.02 0.6 1.5275 6.9 >400  6B 0.02 0.4 1 230 6.5 >400  7B 0.08 1.5 2.5 220 6.5 >400 8B 0.02 1.5 2.5 215 6.6 >400  9B 0.08 0.5 1 230 6.8 >400 10B 0.02 1 2250 6.6 >400 11B 0.01 0.5 1 285 6.8 >400 12B 0.01 0.4 1 270 6.8 >400 13B0.01 0.4 1 225 6.5 >400 14B 0.01 0.6 1.5 270 6.8 >400 15B 0.01 0.5 1 2706.9 >400 16B 0.01 0.5 1 240 6.7 >400 17B 0.01 0.5 1 260 6.8 >400 18B0.01 0.5 1 245 6.8 >400 19B 0.01 0.5 1 240 6.8 20B 0.01 0.5 1 2106.7 >400 21B 0.01 0.5 1 285 7.0 >400 22B 0.2 0.3 1 245 6.9 >400 23B 0.010.6 1.5 230 6.8 >400 24B 0.01 0.5 1.5 270 7.0 >400 25B 0.01 0.5 1 2556.5 >400 26B 0.01 0.5 1.5 240 6.5 >400 27B 0.01 0.5 1 260 6.7 >400 28B0.01 0.5 1 225 6.7 >400 29B 0.01 0.5 1.5 235 6.8 >400 ComparativeExample  5B 0.2 0.7 1.5 195 5.9 295  6B 0.05 2 4 190 5.8 200  7B 1.5 0.51 200 6.4 225  8B 0.01 1 2 200 6.3 330  9B 1.5 0.3 1 175 5.8 230 10B0.02 0.7 1.5 195 6.3 260

TABLE 6 Rolling Ball-Shaped Wear Resistant Member Material Composition(mass %) Sintering Condition HIP Condition Rare Earth Temp. × Time ×Press. Temp. × Time × Press. Sample Si₃N₄ Oxide Al₂O₃ MgO SiC Other (°C.) × (hr) × (MPa) (° C.) × (hr) × (MPa) Example 30B 86 Y₂O₃ 5 2 2 51550 × 6 × 0.01 1500 × 1 × 100 31B 84 Y₂O₃ 5 5 1 5 1550 × 6 × 0.01 1500× 1 × 100 32B 84 Y₂O₃ 5 4 2 5 1550 × 6 × 0.01 1500 × 1 × 100 33B 86 Y₂O₃5 4 2 3 1550 × 6 × 0.01 1500 × 1 × 100 34B 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 11550 × 6 × 0.01 1500 × 1 × 100 ZrO₂ 1 35B 83 Y₂O₃ 5 3.5 1.5 5 TiO₂ 11550 × 6 × 0.01 None ZrO₂ 1 Comparative Example 11B 88 Y₂O₃ 5 1 1 5 1550× 6 × 0.01 1500 × 1 × 100 12B 81 Y₂O₃ 5 7 2 5 1550 × 6 × 0.01 1500 × 1 ×100 13B 86.5 Y₂O₃ 5 3 0.5 5 1550 × 6 × 0.01 1500 × 1 × 100 14B 81 Y₂O₃ 55 4 5 1550 × 6 × 0.01 1500 × 1 × 100 Width of Aggregated Segregation inLiquid Rolling Phase(Grain Boundary Crush Fracture Fatigue PorosityPhase) (μm) Strength Toughness Life of Sample (%) Average Maximum (MPa)(MPa · m^(1/2)) Ball (hr) Example 30B 0.01 0.3 1 270 6.9 >400 31B 0.010.5 1.5 260 6.6 >400 32B 0.01 0.6 1.5 260 6.7 >400 33B 0.01 0.8 1.5 2807.0 >400 34B 0.01 0.4 1 270 6.9 >400 35B 0.3 0.3 1 225 6.8 >400Comparative Example 11B 1.5 0.5 1 190 6.2 225 12B 0.01 0.8 2 205 6.2 28013B 1.2 0.5 1 180 6.5 245 14B 0.01 1.8 3.5 195 6.4 255

As is clear from the results shown in Tables 5–6, in the respectivesilicon nitride rolling balls of Examples each of which was manufacturedin such a manner that the material powder mixture containing specifiedamounts of various additives such as rare earth element, MgAl₂O₄ spinel,SiC or the like was molded and sintered, followed by being subjected toHIP treatment as occasion demanded after completion of the sinteringprocess, the generation of the pore was effectively suppressed therebyto decrease the size of the aggregated segregation in the grain boundaryto be extremely small. Therefore, there could be obtained the siliconnitride rolling balls each having a high crush strength and an excellentdurability such that the rolling fatigue life exceeded 400 hours.

On the other hand, in the silicon nitride sintered bodies as shown inComparative Examples 5B–14B in which the amount of additives such asrare earth components were set to outside the range specified in thepresent invention, even if the sintering operation and the HIP treatmentwere sufficiently performed, the rolling fatigue life of the rollingballs was lowered, and it was confirmed that the sintered bodies ofComparative Examples could not satisfy at least one of the requiredcharacteristics such as porosity, width of the aggregated segregation,three-point bending strength or the like that were specified in thepresent invention.

INDUSTRIAL APPLICABILITY

As described above, according to the silicon nitride wear resistantmember and the method of manufacturing the member of the presentinvention, the material mixture is prepared by adding the predeterminedamounts of the rare earth element, MgAl₂O₄ spinel or the mixture ofmagnesium oxide and aluminum oxide, silicon carbide, and compound of Ti,Zr, Hf or the like to the silicon nitride material powder, so that thesintering property is greatly improved. Therefore, even if the moldedcompact is sintered at a low temperature of 1600° C. or lower, there canbe obtained a silicon nitride wear resistant member having an excellentwear resistance, a high density and a high mechanical strength that areequal to or higher than those of conventional silicon nitride sinteredbody. In particular, the silicon nitride wear resistant member issuitable for a material constituting a rolling bearing member in view ofits excellent rolling life characteristics.

Further, the generation of pores is suppressed and a maximum size(diameter) of pore can be minimized, so that there can be obtained awear resistant member excellent in rolling life characteristics anddurability. Accordingly, when a bearing device is prepared by using thiswear resistant member as rolling bearing member, good sliding/rollingcharacteristics can be maintained for a long time of period, and therecan be provided a rotation machine having excellent operationalreliability and durability.

1. A wear resistant member composed of silicon nitride sintered bodycontaining 2–10 mass% of rare earth element in terms of oxide thereof assintering agent, 2–7 mass % of MgAl₂O₄ spinel, 1–7 mass % of siliconcarbide, and 0.5–5 mass % of at least one element selected from thegroup consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms of oxidethereof, wherein a porosity of said silicon nitride sintered body is 1vol. % or less, a three-point bending strength is 900 MPa or more, afracture toughness is 6.3 MPa·m^(1/2) or more, and a maximum width ofaggregated segregation existing in grain boundary phase of the siliconnitride sintered body is 5 μm or less.
 2. A wear resistant membercomposed of silicon nitride sintered body containing 2–10 mass % of rareearth element in terms of oxide thereof as sintering agent, 2–7 mass %of MgAl₂O₄ spinel, 1–7 mass % of silicon carbide, and 0.5–5 mass % of atleast one element selected from the group consisting of Ti, Zr, Hf, W,Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of saidsilicon nitride sintered body is 1 vol. % or less, a three-point bendingstrength is 900 MPa or more, a fracture toughness is 6.3 MPa·m^(1/2) ormore, and an average width of aggregated segregation existing in grainboundary phase of the silicon nitride sintered body is 2 μm or less. 3.A wear resistant member composed of silicon nitride sintered bodycontaining 2–10 mass % of rare earth element in terms of oxide thereofas sintering agent, 1–2 mass % of magnesium oxide, 2–5 mass % ofaluminum oxide, 1–7 mass % of silicon carbide, and 0.5–5 mass % of atleast one element selected from the group consisting of Ti, Zr, Hf, W,Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of saidsilicon nitride sintered body is 1 vol. % or less, a three-point bendingstrength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m^(1/2)or more, and a maximum width of aggregated segregation existing in grainboundary phase of the silicon nitride sintered body is 5 μm or less. 4.A wear resistant member composed of silicon nitride sintered bodycontaining 2–10 mass % of rare earth element in terms of oxide thereofas sintering agent, 1–2 mass % of magnesium oxide, 2–5 mass % ofaluminum oxide, 1–7 mass % of silicon carbide, and 0.5–5 mass % of atleast one element selected from the group consisting of Ti, Zr, Hf, W,Mo, Ta, Nb and Cr in terms of oxide thereof, wherein a porosity of saidsilicon nitride sintered body is 1 vol. % or less, a three-point bendingstrength is 900 MPa or more, and a fracture toughness is 6.3 MPa·m^(1/2)or more, and an average width of aggregated segregation existing ingrain boundary phase of the silicon nitride sintered body is 2 μm orless.
 5. A method of manufacturing a wear resistant member composed ofsilicon nitride sintered body comprising the steps of: preparing amaterial mixture by adding 2–10 mass % of a rare earth element in termsof the amount of an oxide thereof, 2–7 mass % of MgAl₂O₄ spinel, 1–7mass % of silicon carbide, and 0.5–5 mass % of at least one elementselected from the group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Crin terms of oxide thereof, to a silicon nitride powder containing 1.5mass % or less of oxygen and 90 mass % or more of α-phase type siliconnitride and having an average grain size of 1 μm or less; molding saidmaterial mixture to form a compact; degreasing said compact; andsintering the compact in non-oxidizing atmosphere at a temperature of1,600° C. or lower thereby to form a wear resistant member composed ofsilicon nitride sintered body.
 6. A method of manufacturing a wearresistant member composed of silicon nitride sintered body comprisingthe steps of: preparing a material mixture by adding 2–10 mass % of arare earth element in terms of the amount of an oxide thereof, 1–2 mass% of magnesium oxide, 2–5 mass % of aluminum oxide, 1–7 mass % ofsilicon carbide, and 0.5–5 mass % of at least one element selected fromthe group consisting of Ti, Zr, Hf, W, Mo, Ta, Nb and Cr in terms ofoxide thereof, to a silicon nitride powder containing 1.5 mass % or lessof oxygen and 90 mass % or more of α-phase type silicon nitride andhaving an average grain size of 1 μm or less; molding said materialmixture to form a compact; degreasing said compact; and sintering thecompact in non-oxidizing atmosphere at a temperature of 1,600° C. orlower thereby to form a wear resistant member composed of siliconnitride sintered body.
 7. The method of manufacturing the wear resistantmember composed of silicon nitride sintered body according to claim 5,wherein said method further comprising the step of: conducting a hotisostatic pressing (HIP) treatment to said silicon nitride sintered bodyin non-oxidizing atmosphere of 30 MPa or more at a temperature of 1,600°C. or lower after completion of the sintering step.
 8. The method ofmanufacturing the wear resistant member composed of silicon nitridesintered body according to claim 6, wherein said method furthercomprising the step of: conducting a hot isostatic pressing (HIP)treatment to said silicon nitride sintered body in non-oxidizingatmosphere of 30 MPa or more at a temperature of 1,600° C. or lowerafter completion of the sintering step.